In the US Code, only murder and treason are not subject to statute of limitations, iinm.
But that requires at least a preponderance of the evidence.
Which is exactly what virtually everyone on this thread is saying: if the ‘lucky’ finder of the coins, the current holder of the property, is to have it/them seized, then the seizure should be lawfully executed. The Mint did not bother with a lawful seizure; it merely said, ‘Sorry, Charlie’, and kept the property.